This invention relates generally to machines for punching holes, and particularly to portable, power driven punching machines of the class suitable for working on steel frame members at sites of construction. More particularly, the invention pertains to such punching machines featuring provisions for emitting an aiming beam with a view to punching exactly in position on the work.
Portable, hydraulic punching machines, complete with a pump and an electric drive motor therefor, have been known and used extensively for creating holes in steel beams or strips or like frame members of various cross sectional shapes at construction sites. Such machines are more or less alike in that a punch is moved back and forth by a hydraulic cylinder with respect to a fixed die for punching the work being held against the die.
Difficulties have been encountered in use of such punching machines. In some, if not all, cases, holes must be punched exactly in marked positions on steel frame members. Conventionally, the puncher has had to rely solely on his eyes in aiming at the marking on the work. Such visual aiming has made punching operation very difficult and, sometimes, inaccurate.
Another objection to the prior art concerns the return valve incorporated in the hydraulic cylinder of single acting, spring return type for moving the punch back and forth. Mounted to the piston of the hydraulic cylinder, the return valve functions to release the fluid from the fluid chamber into the spring chamber at the end of the extension stroke of the piston, thereby permitting the piston to be sprung back to the normal position on its contraction stroke.
Conventionally, the return valve was opened by the energy stored in two springs as they were compressed during the extension stroke of the piston. These springs had to be of very heavy make, however, because the return valve had to be opened in opposition to the force of the pressurized fluid acting on the valve head.